3.0 The Integrated Sounding System (ISS)

3.1 Integrated Sounding System Description

The Integrated Sounding System (ISS) was developed jointly by the Surface and Sounding Systems Facility of the National Center for Atmospheric Research Atmospheric Technology Division and the Aeronomy Laboratory of the National Oceanic and Atmospheric Administration. The ISS combines various measurement systems, both in situ and remote, to take advantage of the positive attributes of each. The ISS further integrates the data from the various measurement systems for archival and display.

The ISS consists of four separate subsystems:

The ISS subsystems are integrated physically and digitally. A Sun workstation is the heart of the digital integration. The Sun is the center of the ISS computer network and serves to collect, display, and archive data from each of the subsystems. The Sun is connected to personal computers in both the radiosonde sounding system and the profiler/RASS sounding system via PCNFS. Data from the surface observing station is routed serially via RS-232 directly into the Sun workstation. The Sun can also format data and control data flow for transmission to sites well removed from the ISS site.

Data transmission from an ISS site can be via modem and phone line or via satellite. In the TOGA COARE project, ISS data from island ISS sites in the Pacific were transmitted to the GOES WEST satellite and ultimately relayed to Boulder, Colorado and other project locations. WMO format GTS messages, both sounding and surface, were transmitted in real time from the TOGA COARE ISS sites. These messages were transmitted via the GOES WEST satellite to forecast centers in real time and incorporated into the forecast models. (Those messages and others were also transmitted to Boulder and then relayed back to Townsville, Australia.)

The balloonborne radiosonde navaid (Loran or Omega) sounding system is the standard NCAR "CLASS" sounding system. This sounding system typically uses the Vaisala RS-80 L or the Vaisala RS-80 N radiosondes which use Loran and Omega radionavigation signals respectively for windfinding. Launching configurations can vary depending on the installation. An ISS site may have an enclosed air conditioned launcher, a "bag" launcher, or possibly no launcher at all. The balloon and sonde are secured to structures available before release in the latter case.

The enhanced surface observing station consists of two instrumented towers and a rain gauge. A ten-meter tower is instrumented with wind velocity sensors as well as pressure, temperature, and humidity sensors. A separate two-meter tower is typically instrumented with radiometers. The data are formatted and processed by a Campbell CR10 datalogger. The datalogger is programmable. It is typically configured to generate one-minute average data which are sent via RS-232 to the Sun workstation. Shipboard surface observing station installations require some type of shipboard navaid to correct for ship velocity in the wind velocity measurement.

The 915 MHz wind profiler deployed during TOGA COARE was a three-beam Doppler beam swinging system. The system is now being modified to a five-beam system. The profiler is a long-wavelength Doppler radar which detects the backscattered signals from turbulence-induced refractive index variations. The profiler thus tracks the motion of the turbulent eddies which drift with the mean flow, providing a measurement of the mean wind velocity.

Radial wind velocities are obtained over a variable number of range gates (e.g. 25 to 40) using spectral moment and consensus averaging techniques. Consensus averages can be calculated over varying periods for two vertical modes: one for lower altitude sampling with a high vertical resolution and one for higher altitude sampling with reduced vertical resolution. The raw data are recorded on site in real time to allow for alternative post-processing.

Any shipboard ISS profiler installation requires special attention and adaptation. The microstrip profiler antenna needs to be mounted on a gyrostabilized platform. A three axis accelerometer may be required to measure oscillatory ship motions (backscattered signals are doppler shifted by the motion of the ship) and a GPS receiver is used to determine the speed and heading of the ship. Details of shipboard testing of this system can be found in Carter, et al. (1992).

The radio acoustic sounding system (RASS) utilizes the vertical beam of the profiler to track a broad band acoustic frequency wave front produced by four speakers. (The radar actually tracks refractive index perturbations induced by the acoustic wave traveling at the local speed of sound.) The broad band frequency is used to assure that the Bragg criterion is met. A vertical virtual temperature profile is obtained from the tracking of this wave front (acoustic shell) from the relationship between air density and the propagation velocity (speed of sound) of that wave front.

Vertical virtual temperature profiles with 100 meter resolution can be obtained periodically by the tracking of the generated acoustic signals. A correction can be applied in the real time processing to remove the vertical wind motion from the measured speed of the acoustic shell. Note that the RASS temperature profiles obtained in real time will be drastically affected by hydrometeors. The return from the hydrometeors will invalidate the measurement of vertical wind and the resultant correction will give erroneous profiles. In some cases where there are hydrometeors present, it may be possible to obtain a reliable shell propagation speed in post-processing by using rainfall rate measurements to estimate and remove the hydrometeor fall speeds which would otherwise contaminate the vertical wind measurement.

The ISS sites are housed in a standard 20-foot sea container modified to serve as an equipment shelter and laboratory for project scientists and engineers. The modified sea container houses the Sun workstation, the profiler/RASS computer, the balloonborne sounding system computer, the profiler antenna array, as well as storage for expendables, disks, and tapes. The container may or may not have a balloon launcher attached. The RASS speakers are typically placed around the outside of the container. The surface meteorological instrumentation and Campbell datalogger are outside away from the container.

3.2 ISS Surface Measurement Instrumentation Description

The ISS surface meteorological instrument installation includes several sensors mounted on two separate towers as well as a rain gauge mounted independently. An anemometer is mounted on the top of a ten-meter tower. Temperature and humidity sensors are mounted on the end of a one-meter boom attached to the ten-meter tower at two meters above the surface. The temperature and humidity sensors are aspirated and protected with a radiation shield. The pressure sensor is housed in the box containing the Campbell CR 10 datalogger. That box is mounted on the ten-meter tower at one meter above the surface.

The radiation sensors are mounted on a one meter boom on the top of a separate two-meter tower. The standard ISS radiation sensors include an up-looking solar radiation sensor and a net radiation sensor. In situations which require more complete radiation measurements, additional sensors can be added.

The output from all the sensors is directed to the Campbell datalogger for processing. The Campbell datalogger, which is independently programmable, typically generates one-minute average data which are sent via RS-232 to the ISS Sun workstation. The data input to the Campbell datalogger are five-second sample data.

3.3 ISS Surface Measurement Instrumentation Specifications

3.3.1 Pressure Measurement

The surface pressure sensor used in the ISS installation is either a a Vaisala PTA427 or PTA427A pressure sensor. The PTA427 sensor pressure range is 800 to 1060mb while the PTA427A sensor pressure range is 600 to 1060mb. These sensors have an accuracy of +/- 0.5mb and +/- 0.8mb respectively. They are both silicon capacitive pressure sensors patented by Vaisala. Both are temperature compensated and produce a linear voltage output over the full operating range. In order to interface with the Campbell datalogger, a 2:1 voltage divider is incorporated into the cable from the pressure sensor.

3.3.2 Temperature and Humidity Measurement

The temperature and humidity sensors are contained in a Vaisala HMP35C instrument probe. The actual sensors are a Fenwal Electronics UUT5J1 thermistor and a Vaisala "humicap" capacitive relative humidity sensor. The temperature sensor accuracy is +/- 0.4 degrees C over the range -33 to +48 degrees C. The accuracy of the humidity sensor against field references is approximately +/- 2% with a long term stability of better than 1% RH per year. The HMP35C sensor probe is protected and ventilated by an RM Young aspirated radiation shield model number 43-408.

3.3.3 Wind Measurement

Wind speed and direction are measured with an R.M. Young 05103 Wind Monitor. The monitor is a propeller wind vane with a 0.9 m/s threshold for wind speed and a 60 m/s maximum. Wind direction is measured using a 360 degree mechanical precision conductive potentiometer. Direction measurements have a threshold of 1.0 m/s at a 10 degree displacement and 1.5 m/s at a 5 degree displacement. The potentiometer is 10 K-ohm, with a life expectancy of 50 million revolutions and has a 0.25% linearity through the entire range.

3.3.4 Radiation Measurements

The standard ISS radiation measurements are incoming solar radiation and net radiation. The incoming solar radiation measurement is made with a LI-COR LI200S pyranometer. The net radiation sensor is a Radiation and Energy Balance Systems, Inc. Fritschen net radiometer. The net radiation sensor is a single sensor with up-looking and down-looking hemispheric sensors mounted back-to-back at the end of a 0.5 meter boom.

The LI-COR LI200S pyranometer utilizes a silicon photodiode which has a spectral response in the wavelength band from about 0.4 um to about 1.2 um. The LI200S is calibrated against an Eppley Precision Spectral Pyranometer (PSP). The calibration is periodically checked. The absolute error of the calibration is + / - 5.0% maximum with the typical error being + / - 3.0%. The cosine response has been corrected up to an 80 degree incidence angle.

The Radiation and Energy Balance Systems, Inc. Fritschen net radiometer, measures the sum of all incoming radiation (direct solar, diffuse solar, longwave skylight) minus the sum of all outgoing radiation (reflected radiation and terrestrial longwave radiation). A single data stream, the difference of incoming and outgoing radiation, is recorded from the Radiation and Energy Balance Systems, Inc. Fritschen net radiometer. The wavelength range of this instrument covers both the shortwave and longwave bands.

When more precise radiation measurements are required, three separate radiation measurements can be made in the surface meteorological installation, shortwave, longwave, and net radiation. Both the shortwave and longwave radiation measurements are then made with pairs of Eppley radiometers, one up-looking and one down-looking radiometer in each pair. The net radiation measurement is again made with the Radiation and Energy Balance Systems, Inc. Fritschen net radiometer.

The shortwave radiation measurements are made with Eppley PSP pyranometers. The wavelength range of these instruments is 0.3 to 3.0 um. Separate data streams are recorded from both the up-looking and down-looking pyranometers. The longwave radiation measurements are made with Eppley pyrgeometers. The wavelength range of these instruments is 4.0 to 50.0 um. As with the pyranometers, separate data streams are recorded from both the up-looking and down-looking pyrgeometers.

3.3.5 Rain Measurement

A Texas Electronics TE525 tipping bucket rain gauge is used at all land based ISS sites for measurement of rainfall. The rain gauge resolution is 0.254 mm. The gauge is typically positioned 0.5 meters above the ground about 7 or 8 meters from the ten-meter tower. Shipboard rain measurements can be made with a Scientific Technology Inc. optical rain gauge, STI ORG-100. These STI rain gauges are currently not available as standard ISS furnished equipment. At present they would have to be purchased by the project. They may become available as standard equipment at a later time as funding permits.

3.4 ISS Balloonborne Sounding System Description

The ISS balloonborne sounding system is the standard NCAR "CLASS" sounding system (See Chapter addressing the CLASS system for complete information). The integration of the surface meteorological data into the sounding data is somewhat different in the ISS. In the NCAR ISS configuration, that surface data are made available to the "CLASS" sounding system through the ISS computer network via PCNFS. (The surface data are first transferred from the Campbell datalogger via RS-232 to the ISS Sun workstation.)

3.4.1 Radiosonde Deployment

There are differing launch configurations possible at any ISS site. There can be an enclosed air conditioned launcher or a "bag" launcher can be used where a heavy vinyl tarp contains and protects the balloon prior to launch. A "bag" launcher is typically used on shipboard ISS installations. In addition, a site can function without a launcher, in which case, the balloon and sonde are tied outside the ISS container and then released.

3.5 ISS Wind Profiler and RASS Specifications

Up to four sets of radar parameters can be defined for the ISS wind profiler and RASS system. These are set up interactively by the operator. The operator enters values for the inter-pulse period (IPP), the pulse width (PW), the number of heights (NHTS), the time from the first transmitted pulse to the first sampled height (DELAY), and the spacing between the sampled heights (SPACE). The four pulse widths available define the vertical resolution of the wind profiler and RASS. Those pulse widths are as follows:

The pulse width also defines the amount of energy transmitted by the radar and in most cases is reflected in the maximum altitude coverage. Another key factor in altitude coverage is the availability of scatterers to provide refractive index irregularities within the sample volume. Most variations in refractive index depend on the variation of temperature and humidity caused by turbulent eddies that are in a size range on the order of one-half the radar wavelength.

The following tables summarize the wind profiler and RASS system characteristics. Note that on the wind profiler height resolution entries, resolutions are given for two modes, a low altitude mode (high resolution) and a high altitude mode (low resolution). The height to which the profiler can measure wind velocities is highly dependent on the atmospheric characteristics.

TABLE  12  Wind Profiler Typical Performance Specifications
Radar Frequency       915 MHz               
Radar Wavelength      0.33 meters           
Mean Power            7.0 W                 
Antenna size          4.0 square meters     
Beam Width            9.0 degrees           
Minimum Height        0.12 km AGL           
Maximum Height        2 to 5 km AGL         
Height Resolution 

Low altitude mode:
60 & 100 meters
High altitude mode: 250 & 500 meters Wind Speed < 1.0 m/s Wind Direction < 1.0 degree ------------------------------------------
TABLE  13  RASS Specifications
Acoustic Frequency      2000 Hz      
Acoustic Power:
40.0 W Total Electrical Input Acoustic Beamwidth 8.0 degrees -----------------------------------

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